This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. The long-term objectives of the proposed research are to elucidate the biochemical mechanisms underlying the circadian clock in Chlamydomonas reinhardtii. This unicellular, eukaryotic green alga serves as a model system for a number of cell processes in animals and plants. Understanding its circadian clock will contribute to an understanding of circadian clock evolution and thus also help to better understand the circadian clock in humans. The strategy of achieving the long-term objectives is to identify the photoreceptor(s) responsible for adjusting the circadian clock to the daily light/dark cycles. If successful, the photoreceptors can be used as a handle to investigate the signal transduction chain(s) that convey the information to the central oscillator and possibly the components of the oscillator itself. C. reinhardtii provides an advantage over other model organisms such as Arabidopsis thaliana because it contains only a limited number of photoreceptors. They fall into three classes: cryptochromes, rhodopsins, and ototropins. Phototropins are blue-light photoreceptors found in higher plants and green algae (Briggs and Christie 2002). Whether phototropins are involved in lightentrainment of the circadian clock is not completely understood yet. Cryptochromes on the other hand are blue-light photoreceptors that were shown to be involved in clock entrainment in higher plants and insects (Cashmore 2003). They are also found in mammals but serve as components of the pacemaker that creates the circadian oscillation in this group of organisms. In fungi as exemplified by Neurospora crassa, cryptochrome seems not to be involved in the circadian clock.